In recent years, pressure vessels and particularly pressure vessels which are used under wide temperature ranges, have been constructed of various filament reinforced composites. As a simple example, many pressure vessels are constructed with reinforced plastic composites of epoxy resins and fibrous materials such as glass, boron, carbon or the like. The reinforced plastic composite materials are highly effective in the formation of pressure vessels due to the fact that they have a high strength capable of withstanding large pressure and temperature changes, and they are of significantly lighter weight than pressure vessels formed of metals such as steel, aluminum, or the like. Reinforced composite materials are frequently used for vessels exposed to outer space environments, such as on rockets, and are proposed now for use in space station equipment. The use of a filament wound composite material is also ideal for manufacturing very large vessels, that is, vessels over 10 feet in diameter and vessels of even larger size. However, vessels of this type lack the requisite barrier properties without further treatment or processing.
More particularly, one of the disadvantages of reinforced composite vessels is the existence of a microporosity which becomes pronounced at low temperatures. The microporosity can result during the initial manufacturing operations of fiber compaction and resin curing. Porosity can also result from cyclic pressure loading of the composite structures in service. However, in the minds of most researchers, the thermal properties and the mechanical properties of the fibers which are used and of the resin matrices which are employed are so widely divergent that micro-cracking between the fiber and the matrix is inevitable. When the vessel is used to contain cryogenic fluids, such as hydrogen fuel at -423.degree. F., thermal micro-cracking becomes increasingly problematic. Liquid hydrogen is a particularly difficult substance to contain under low temperature conditions. Due to the fact that the hydrogen molecule is a very small molecule, there is a great tendency for this fluid to weep through the vessel wall, thus resulting in a loss of material.
In order to overcome the microporosity problem, others have attempted to use liners in these reinforced composite pressure vessels. The search for an effective barrier material for filament wound pressure vessels started as early as the 1950's when rubber bladders were used simply to allow the vessel to be hydrostatically proof-tested. Thereafter, others have attempted to use the rubber bladders as a type of liner. Some fabricators in the design of pressure vessels, such as water tanks, used seamless spun aluminum liners that were over-wound with filaments and resin binders and then left in place during service operation. There were many research efforts to find suitable film barrier materials using Mylar, Tedlar, Kapton or acrylonitrile-butadiene to prevent leakage. However, none of the above described materials possesses the required elasticity at cryogenic temperatures, such as -423.degree. F., the liquidation point of hydrogen.
There are many liquid polymeric formulations which can be sprayed in place, rolled on in liquid form, or applied by brush. However, all of these polymers and their application methods result in the inclusion of air bubbles and pinholes in the resultant liner. Most frequently, these air bubbles and pinholes cannot be seen and only appear under the stress of extreme temperature conditions. Further, these liners cannot be sealed air or liquid tight when pressure tested.
In addition, other films have been proposed which use an adhesive B-stage film. These films do not possess sufficiently elastic properties at cryogenic temperatures and also require high temperatures and pressures to fully cure the film. To apply the requisite pressures and temperatures, the structure is usually placed in a high temperature pressurizing device, such as an autoclave. Such an approach is impractical, however, when working with large tank structures, such as those having a diameter of 50 feet or greater, which will not readily fit into an autoclave. In addition, even when using an autoclave, the pressure may not be sufficient to remove air bubbles from beneath the film.
Thus, there is a need for an effective technique for sealing reinforced composite pressure vessels, particularly when subjected to pressure and temperature variations of the type encountered in space applications. There is a need for a barrier liner material for reinforced composite fluid vessels which remains fully sealed, even under pressurized cryogenic conditions. In addition, it would be desirable to provide an easy and effective method of applying the liner to a vessel wall, even for very large tank structures.